1. Field of the Invention
The invention relates generally to projection systems with compensation of intensity variations and compensation elements therefor.
The invention also relates to a projection objective for imaging a pattern arranged in the object plane of the projection objective into the image plane of the projection objective, and to an optical component having a substrate in which at least one substrate surface is covered with an interference layer system. The optical component may be provided in particular for incorporation into a catadioptric or dioptric projection objective or be incorporated into the latter.
The invention also concerns a projection exposure apparatus, in particular for immersion lithography, such as is specified in the US patent applications bearing the official identification 60/592,208, 60/530,978, 60/591,775 or 60/612,823, which were not published before the priority date. The immersion medium therein is a liquid, preferably water.
The invention additionally concerns a projection exposure apparatus, in particular for near field lithography—synonymously SIL lithography (solid immersion lens)—such as is specified for example in the application document US 2003/174301 A1.
2. Description of the Related Art
Projection exposure apparatuses for microlithography are used for fabricating semiconductor components and other finely patterned devices. They serve for projecting patterns from photomasks or reticles, referred to hereinafter generally as masks or reticles, onto an object coated with a radiation-sensitive layer, for example onto a semiconductor wafer coated with photoresist, with very high resolution on a demagnifying scale.
In the process for photolithographically patterning semiconductor components, it is of crucial importance that all structure directions, in particular horizontal and vertical structures, be imaged with essentially the same imaging quality. The imaging quality of lithography objectives is determined not only by the correction state of the aberrations, but also by the profile of the intensity over the field and over the pupil of each field point. The profiles in field and pupil are intended to be as uniform as possible. Local intensity modulations in the image plane are to be avoided inter alia because the binary resist materials that are customary nowadays have a strongly nonlinear characteristic curve of sensitivity which, to a first approximation, can be modeled by the fact that an exposure takes place above an intensity threshold, while no exposure takes place below the intensity threshold. As a result, the spatial intensity profile directly influences the width of the structures produced on the semiconductor component. The more uniform the linewidth over the field and over different structure directions, the higher may be the clock frequency when using the finished semiconductor component and correspondingly the performance and the price of the finished semiconductor component. Therefore, the variation of the critical dimensions (CD variation) is an important quality criterion.
The variation of the critical dimensions may have different, alternatively or cumulatively effective causes. Especially in systems that operate with polarized light, reflectivity or transmission properties of interference layer systems that are dependent on the angle of incidence may lead to nonuniformities of the intensity over field and pupil. This problem area may occur in the case of purely refractive (dioptric) imaging systems. The abovementioned causes may occur particularly in catadioptric projection objectives, that is to say in those systems in which refractive and reflective components, in particular lenses and imaging mirrors, are combined. When utilizing imaging mirror surfaces, it is advantageous to use beam splitters if an imaging that is free of obscuration and free of vignetting is to be achieved. Systems with geometrical beam splitting and also systems with physical beam splitting, for example with polarization beam splitting, are possible. The use of mirror surfaces in such projection objectives may contribute to CD variations arising during imaging.
When using synthetic quartz glass and fluoride crystals, such as calcium fluoride, it must furthermore be taken into consideration that these materials are birefringent. They may bring about polarization-altering effects on account of induced and/or intrinsic birefringence on the light passing through, which effects likewise contribute to CD variations arising. Finally, material defects, e.g. scattering centers or striations in transparent components, may also lead to nonuniformities of the intensity distribution.
Also, absorbent optical elements, in particular immersion media, in particular immersion liquids, solid immersion lenses (SIL) or other, refractive optical materials having a high refractive index, lead to a field- and pupil-dependent transmission distribution and thus to an error in the transmission profile.
International patent application PCT/EP2004/001779 filed by the applicant on Feb. 24, 2004 and claiming priority from German patent application DE 102004002634.3 filed on Jan. 19, 2004 by the applicant discloses a microlithographic exposure apparatus designed for immersion lithography where the projection objective includes a transmission filter which is designed and arranged in the projection objective in such a way that rays which enter an immersion interspace formed between the last optical element on the image side and the image plane of the projection objective and filled with an immersion liquid from the last optical element on the image side at an angle of incidence are attenuated more strongly the smaller the angle of incidence is. The complete disclosure of that PCT application is incorporated into the present application by reference.
As the period over which the projection exposure apparatus is used increases, said transmission distributions vary on account of an alteration of the transmission behavior of the element. This requires maintenance of the projection exposure apparatus.
It is known that the transmission behavior of optical systems can be altered by means of a pupil filtering or apodization, interventions being possible both on the intensity distribution of the radiation passing through and on the phase angle of the radiation. The U.S. Pat. No. 5,222,112 describes a projection system for extreme ultraviolet light (EUV) that operates exclusively with mirror components, in the case of which degradation of the imaging performance may occur on account of different reflectances for s- and p-polarized light on multiply coated mirrors. A convex mirror arranged in the region of a pupil surface of the projection objective has a rotationally symmetrical reflectivity distribution with reflectivity that decreases to the edge of the mirror in order to improve the imaging properties of the system. A transmission filter that is transmissive for soft x-rays, with a corresponding, rotationally symmetrical transmission function, is mentioned as an alternative. A number of possibilities are mentioned for producing a mirror with a sufficiently great spatial modulation of the reflectance. One possibility consists in designing the reflective, multilayer interference layer system in such a way that the layer thicknesses of the individual layers increase or decrease continuously from the center to the edge (graded thickness multilayer). Other possibilities discussed include altering the number of layer pairs in the multilayer coating, ion implantation after the fabrication of the multilayer coating, or the production of an absorber layer with a suitable distribution of the layer thickness on the multilayer coating. The mirror arranged in the region of a pupil surface in any event acts as a spatial frequency filter. No detailed information is given on the construction of the reflection coating.
The international patent application WO 02/077692 A1 presents a method for producing optical systems which is intended to make it possible for example to provide projection objectives with extremely small wavefront aberrations even if the lenses have to a greater or lesser extent inhomogeneities of the refractive index or defects of form. The method comprises a measuring step, in which the refractive index distribution of an optical material used for producing a lens is measured, and a surface measuring step, in which the surface form of a lens is determined. An optical error or a wavefront error of the lens is determined on the basis of the measurement results. On the basis of the calculation results, an optical coating is produced on the lens, said optical coating having a predetermined thickness distribution which is suitable for minimizing the wavefront error. The coating is thus designed for establishing a phase that is as homogeneous as possible at the coated optical component.